It is now apparent that although current antiretroviral drugs can reduce HIV-1 replication to undetectable levels in plasma, they cannot lead to a cure of infection, as virus levels rebound rapidly after cessation of therapy. This rebound is the result of reservoirs of replication-competent viruses that resume active replication following termination of antiviral drugs. The best-described and possibly largest viral reservoir is that of latently infected CD4+ memory T cells. A major gap in our knowledge of HIV-1 infection is the identification of mechanisms involved in establishing and maintaining latent infection in these cells. The research proposed in this application is designed to begin to fill in this gap. The fundamental feature of latent infection of memory CD4+ T cells is the transcriptional shut-off of the integrated provirus. To allow latent infection to be established, it is likely that key cellular factors involved in transcription directed of the HIV-1 long terminal repeat (LTR) sequences must be down-regulated after proviral integration. Additionally, it is likely that the induction of T cell quiescence after proviral integration leads to alterations of the HIV-1 LTR that contribute to repression of transcription. Transcription of the HIV-1 LTR is dependent upon a cellular factor known as TAK/P-TEFb that mediates Tat transactivation function. TAK/P-TEFb is a cellular protein kinase composed of CDK9 as the catalytic subunit and cyclin T1 as a regulatory subunit. We propose here studies to address the hypothesis that mechanisms exist in CD4+ T cells that down-regulate TAK/P-TEFb, and such negative regulation is important to latent infection. We will also investigate how T cell quiescence influences HIV-1 LTR expression. We will test the hypothesis that the viral Nef protein counteracts the program of T cell quiescence. We will also explore the hypothesis that quiescence leads to alterations in the integrated provirus that repress HIV-1 LTR transcription. Successful completion of this research will provide new information about mechanisms involved in the establishment and maintenance of latent infection in CD4+ T cells. This information may serve as the basis for developing novel strategies to reduce viral reservoirs. The reduction of viral reservoirs is likely to have therapeutic benefit in HIV-infected individuals.